This research is aimed at assessing comparatively the potential carcinogenic potency of tritiated water versus tritiated DNA, RNA and protein precursors, namely thymidine, uridine, or leucine, in relation to their specific patterns and sites of incorporation in C57 Black/6M mice. For this purpose the basic (spontaneous) tumor spectrum in control animals will be defined as a multiple competitive tumor model, from which the variation in the tumor spectra possibly associated with exposure to various tritiated compounds will be evaluated, both quantitatively and qualitatively. In addition, it is planned to investigate the potential cumulative genetic injury resulting from exposure of male mice to tritiated water or tritiated thymidine, prior to sibling mating, during 25 successive generations. The incorporation patterns of tritiated precursors of DNA, RNA and proteins versus tritiated water in selected target organs have been investigated, in an attempt to correlate the possibly induced tumor spectra with various sites of tritium incorporation.